1. Field of the Invention
This invention relates to a cover for accommodating an air bag in an air bag system designed to confine a passenger in a seat upon the occurrence of a collision of an object running at a high speed such as an automobile or the like (hereafter, high-speed running object), and to prevent damage to the passenger from a secondary collision with a steering wheel or an instrument panel of the automobile.
2. Description of the Related Art
An air bag system is generally comprised of a sensor apparatus for sensing a collision of the high-speed running object and an air bag apparatus comprising an air bag, an inflator for generating a gas for expanding the air bag, a cover for accommodating the air bag and the inflator fixed thereto, a retainer for retaining the inflator and the cover in the body of the high-speed running object, and arranged at a position opposite to that of a passenger in the high-speed running object. Accordingly, when a collision occurs, a gas is instantaneously generated by the inflator, to thus fill the air bag connected to the inflator and folded up in a space formed between the inflator, the retainer and the cover, and the cover is opened by a pressure of the gas fed into the air bag, whereby the passenger is confined in a seat because the air bag is expelled from the cover and is instantaneously expanded toward a front side of the passenger, and thus injury to the passenger by a collision with a steering wheel or an instrument panel is prevented.
Accordingly, a cover used in the air bag apparatus must easily allow an instantaneous expansion of the air bag without a scattering of broken pieces of the cover, which may injure the passenger, when the collision occurs and the inflator is operated.
Various type of covers for the air bag apparatus are proposed in known references, e.g., Japanese Unexamined Patent Publications (Kokai) No. 50-127336, No. 55-110643, No. 52-116537, No. 1-202550, or the like.
Namely, the Japanese Unexamined Patent Publications (Kokai) No. 50-127336 and No. 55-110643 disclose a cover made of an elastic material such as a urethane resin and having a structure in which a reinforcing material such as a fiber net, a metal wire net or the like is embedded in all portions other than a portion of the cover to be broken open. Further, a metal plate may be used as the reinforcing material in this type of the cover. Namely, the resin used in the above cover is essentially fragile, and thus it is necessary to embed the reinforcing material in the resin to thereby prevent a scattering of the broken pieces. This structure is disadvantageous in that much time is required to arrange the reinforcing material in the resin when manufacturing the cover, and it is difficult to precisely embed the reinforcing material at a predetermined position in the cover. Further, when the urethane requiring reaction injection molding is used as the resin, much time is needed for a sufficient progress of the reaction in a mold, and thus this type of cover has the disadvantages of a slower production speed and lower production yield. The above disadvantages are fatal from the viewpoint of the automobile industry, because the production of automobiles in Japan has reached around 10.sup.7 cars per year.
Japanese Unexamined Patent Publication (Kokai) No. 52-116537 discloses a cover made of a thermoplastic elastomer having a brittle temperature of -50.degree. C. or lower and a bending stiffness of between 1,000 kg/cm.sup.2 and 3,000 kg/cm.sup.2, and a portion thereof to be broken having a fragile structure. Since this type of cover has no reinforcing material, incorporated therein, the productivity is improved, but the bending stiffness of this thermoplastic elastomer is high, i.e., between 1,000 kg/cm.sup.2 and 3,000 kg/cm.sup.2, and accordingly, an inflator having a bigger volume must be used to break open the cover, due to the high bending stiffness thereof. Further, the obtained cover has a high hardness, due to the use of the thermoplastic elastomer having the high bending stiffness, and accordingly, this cover cannot provide a suitably soft touch when coming into contact with the passenger, and provides little of the psychological serenity of the driver, etc., essential to safe driving. Further, when the high-speed running object further collides with another body, due to a skidding thereof or the like after the high-speed running object is subjected to a first collision and the air bag is expanded, the passenger could be injured by hard broken edges of the cover opened by the first collision, and accordingly, this cover is not preferable for practical use.
Japanese Unexamined Patent Publication (Kokai) No. 1-202550 discloses a cover obtained by an integral molding of a surface layer of a soft resin having a hardness according to JIS K 6301 TypeA of between 30 and 70, and a core layer of a hard resin having an elasticity and having slits to be further provided in the core layer. Although this cover can preferably provide a suitably soft touch against the passenger, nevertheless, since it is necessary to intermittently mold two layers i.e., the surface layer and the core layer to make this cover, this cover has a disadvantage in that a complicated and expensive injection machine having two injection mechanisms must be used.
U.S. Pat. No. 4,895,389 discloses a rupturable cover including two door sections having a generally S-shaped hinge. This cover, however, is formed from e.g., Du Pont's "Hytrel" copolyether ester elastomer, and accordingly, the hardness of this cover is high and there is a possibility that a passenger will be injured by broken pieces of this cover.
U.S. Pat. No. 5,013,065 published on May 7, 1991, discloses an air bag having a cover consisting of a one-piece injection molding of a thermoplastic elastomer, but the thermoplastic elastomer used in this cover has a thermal stability of from -40.degree. C. to 85.degree. C., and accordingly, this cover cannot be used for an air bag apparatus in a high-speed running object in which the temperature can reach around 90.degree. C.
The following six requirements must be satisfied by a cover of an air bag apparatus. First, the reliability of the opening of the air bag. Namely, when the air bag apparatus is operated, the cover can be opened without scattering broken pieces of the cover, which may injure the passenger, and allows an instantaneous expelling of the air bag. Further, preferably the passenger is not injured by already broken portions of the cover when the high-speed running object further collides with another body due to a skidding thereof or the like after the high-speed running object is subjected to a first collision and the air bag is expanded.
Second, the cover must be able to be operated In a broad range of temperatures. Namely, a temperature of the cover can be raised or lowered between -40.degree. C. and +90.degree. C., depending on an area, season or circumstance in which a car is used. Accordingly, the cover must not become brittle and lose the reliability of an expansion thereof at a lower temperature, or become too soft and easily deformed at a higher temperature.
Third, the cover must have a good resistance to heat and light. Namely, the cover may be arranged at a position irradiated by direct sunshine passing through a front glass of the car in which the air bag system is used, and may be used for five years, or even ten years or more, and accordingly, the cover must not age during the above long use, and thus lose the reliability of the opening thereof.
Fourth, a process of manufacturing the cover must have a high productivity and a lower cost. This requirement is especially important with regard to an air bag apparatus used in cars made by mass production.
Fifth, the cover must have a good touch. Namely, preferably the cover applies a suitably soft touch when the passenger is in contact with the cover, and thus provides a psychological serenity important to safe-driving.
Sixth, the cover must have as light a weight and as compact a size as possible. Namely, in view of fuel costs, a space for accommodating the air bag, and other factors, the cover itself, and the air bag apparatus as a whole, must have a light weight and compact size.